This application is a continuation-in-part of pending application Ser. No. 562,568, filed Mar. 27, 1975 now abandoned. In particular, this invention relates to apparatus for reducing the electrostatic field radiated by conventional diathermy apparatus in the heat treatment of internal body tissues without significant skin surface heating effects.
Medical diathermy involves the use of high frequency electric currents for the therapeutic heat treatment of body tissues. The radio frequencies employed are sufficiently high so that nerves and muscles are not adversely stimulated. The applied intensity is sufficiently low so that the temperature developed in the tissue is below that required to destroy the tissue or impair its vitality.
Radio frequency currents generated by the diathermy machine are controllably applied to the patient via the diathermy applicator head, which is designed to transcutaneously transmit high frequency energy to deep afflicted tissue areas. However, the radiated energy must penetrate a layer of skin and fat before reaching the targeted muscle tissue. The fat layer has little vasculation and cooling capacity, and in addition it has a lower specific heat than lower tissues, resulting in a greater temperature rise per unit heat input. Thus, with typical applicator heads, painful heating of the subcutaneous fat layer is not altogether uncommon. Irritating and painful skin burns are also regular incidences of conventional diathermy treatment. In response to initial heating of the skin, perspiration occurs. Perspiration contains a relatively large amount of sodium chloride in solution, and thus perspiration is highly conductive electrically. Steam burns occur when circulating currents in the salt water raise the temperature to the vaporization point.
It has been found that the deleterious surface heating effects just described are primarily caused by the electrostatic fields radiated by conventional diathermy applicators. The matter is discussed, for example, in an article entitled "Therapeutic Application of Electromagnetic Power", which appears at page 55 of Proceedings of the I.E.E.E., Vol. 62, No. 1, Jan., 1974.
The radiated electromagnetic field, however does not cause harmful burning of surface tissues. Instead, it penetrates deeply enough to provide relative heating in the muscle tissues. This deep heating effect results from the fact that the electromagnetic field lines are tangential to the tissue interfaces rather than perpendicular, so that boundary conditions do not significantly affect or cause surface heating effects. Thus the therapeutic deep heating benefits of diathermy treatment are primarily caused by electromagnetic energy. This conclusion refutes learning in the prior art as discussed, for example, at page 285 of a book entitled Second Edition of Therapeutic Heat and Cold, edited by Sydney Licht, M.D., published by Waverly Press Inc., Baltimore, Md, 1965.
The instant invention comprises a shield for diathermy heads which greatly attenuates radiated electrostatic fields while permitting the transmission of electromagnetic energy therethrough, and a method for heat treating internal tissues with a diathermy apparatus. The shield comprises a plurality of non-magnetic metallic (preferably copper) strips which are located interiorly of a coplanar, generally circular loop. Each of the strips is orientated generally perpendicular with respect to the current carrying coils in the diathermy applicator head on which the shield is mounted. The width of the strips is quite narrow with respect to the wavelength of the energy radiated by the diathermy applicator head. Each of the strips which extends generally radially outwardly from the vicinity of the loop center, terminating at the loop and being electrically connected thereto. The loop, which is sufficiently large to prevent interference with the electromagnetic field radiated therethrough, provides further electrostatic shielding effects by preventing fringing of the electrostatic field.
In an alternative embodiment of this invention, two preferably circular shields, constructed generally as mentioned above, are stacked together. One of the circular shields may be mounted for relative rotation with respect to the other, so that it can be rotated as desired to control the amount of electrostatic energy passing through to the patient. In a first position the strips in each circular shield are aligned to permit some electrostatic energy to be transmitted to the patient. In a closed position, however, the strips of the first circular shield are aligned between the strips of the second so that transmission (to the patient) of electrostatic field energy is substantially reduced.
The method disclosed by the instant invention comprises the steps of generating radio frequency electric currents to thereby produce electrostatic and electromagnetic energy that can be directed to the body tissue to be treated via the diathermy applicator head. The method also includes the step of attenuating at least a portion of the radiated electrostatic energy before it reaches the exterior skin surface in order to minimize the skin heating effects associated with the diathermy treatment. Finally, the method calls for the transcutaneous transmission of the radiated electromagnetic energy substantially unimpeded to the body tissues to be treated. The attenuation of the electrostatic energy can be best accomplished by using an electrostatic shield mounted on the diathermy applicator head.
It is therefore a fundamental object of the method and apparatus of this invention to reduce the surface heating effects (such as skin burning) associated with conventional shortwave diathermy applicator heads while at the same time facilitating therapeutic muscle treatment. Accordingly, the present invention attenuates the transmission of deleterious electrostatic energy to the patient receiving treatment while permitting the passage of the more desirable electromagnetic energy substantially unimpeded.
Another object of this invention is to provide a shield of the character described which minimizes electromagnetic energy losses. It is a feature of this invention that the width of the conductive strips mentioned above is extremely narrow when compared to the wavelength of the emitted energy. The shorted turn effect is reduced by leaving a space at the center of the loop into which the strips do not extend. Finally, a loop surrounds the strips to prevent electrostatic fringing. Together these features promote the overall efficiency of the shield, facilitating the transmission of electromagnetic energy and impeding the passage of electrostatic energy.
Yet another object of this invention is to provide an electrostatic shield of the character described which can easily be used with a wide variety of diathermy applicator heads. It is a feature of this invention that the aforementioned strips and loop can be attached to the existing applicator heads by glue or adhesive. Also, the strips and loop can be constructed by appropriate metal depositing techniques. For example, the pattern can be metalized on the plastic cover of conventional applicator heads.
Still another object of this invention is to provide means for reducing the detuning effect experienced by diathermy applicator heads when the heads are applied to the desired "load" (which comprises the patient's body). It is a feature of this invention that undesirable electrostatic (capacitive) coupling between the patient's body and the applicator head is significantly reduced when the shield is installed. Thus, when an applicator head incorporating the instant invention is positioned adjacent the patient's body, it will not be severely detuned from resonance. The latter characteristic is a significant synergistic effect exhibited by the instant invention. Thus, when the shield is installed on conventional diathermy heads, operating ease and efficiency are increased and undesirable side effects are reduced.
Another object of this invention is to provide a uniquely constructed "variable" shield for diathermy applicator heads which may be adjusted by the patient (or the operator of the diathermy apparatus) to control a preselected amount of electrostatic energy passing therethrough.
Other and further objects of this invention will become apparent in the course of the following description.